The invention relates to an apparatus for heat treatment of continuously moving synthetic yarns of fibers. Heating yarn or fibers by bringing the yarns into contact with a heated plate or roll is well-known in the art. The plate or roll can be heated by an electrical heater or it may be part of a closed vessel which is supplied with steam or in which steam is generated and condenses at the wall of the vessel along which the fiber is drawn. Improved efficiency with respect to heat transfer can be achieved by allowing the steam to condense at the fiber itself so that condensation heat is transmitted directly to the yarn rather than via a heated wall or roll. German Auslegeschrift No. 17 60 956 shows an apparatus of this kind where the yarn passes through an elongated vessel filled with saturated steam which condenses on the moving yarn and transfers its heat to the yarn. In order to prevent the steam from escaping through the inlet and outlet ports of the vessel where the yarns enters and leaves the vessel, a labyrinth seal is provided at each end of the vessel and the central portion of each labyrinth seal is supplied with air under pressure with such pressure corresponding approximately to the pressure of the steam within the vessel.
It is an object of the present invention to avoid the noise caused by the pressurized air escaping from labyrinth seals to the outside and furthermore to eliminate the loss of energy caused by continuously escaping pressurized air.